(a) 21Sc3+=1s2,2s2,2p6,3s2,3p6
It is colourless due to absence of unpaired electrons in d-subshell.
(b) 26Fe2+=1s2,2s2,2p6,3s2,3p63d6
It has colour due to presence of four unpaired electrons in d-subshell.
(c)22Ti3+=1s2,2s2,2p6,3s2,3p63d1
It has colour due to presence of one unpaired electrons in d-subshell.
(d) 25Mn2+=1s2,2s2,2p6,3s2,3p63d5
It has colour due to presence of 5 unpaired electrons in d-subshell.
Hence, option A is correct.